springMVC源码分析--HttpMessageConverter参数read操作（二）

上一篇博客 springMVC源码分析--HttpMessageConverter数据转化（一）中我们简单介绍了一下HttpMessageConverter接口提供的几个方法，主要有以下几个方法：

（1）canRead 是否可以读

（2）canWrite 是否可以写

（3）read() 读数据

（4）write() 写数据

接下来我们介绍一下读取数据的处理操作。

首先要去重温一下参数处理器需要做的操作springMVC源码分析--RequestParamMethodArgumentResolver参数解析器（三），AbstractNamedValueMethodArgumentResolver中的resolveArgument方法，解析参数值

public final Object resolveArgument(MethodParameter parameter, ModelAndViewContainer mavContainer,NativeWebRequest webRequest, WebDataBinderFactory binderFactory) throws Exception {.......Object arg = resolveName(namedValueInfo.name, parameter, webRequest);......}

简单的提交数据的示例：

 @RequestMapping(value="/save")    public String saveProduct( String name,@RequestBody String description,@RequestBody float price, Model model)   .......}

我们可以在HandlerMethodArgumentResolver的子类RequestResponseBodyMethodProcessor的方法resolveArgument看到如下处理，这里就是对提交的参数的处理

public Object resolveArgument(MethodParameter parameter, ModelAndViewContainer mavContainer,NativeWebRequest webRequest, WebDataBinderFactory binderFactory) throws Exception {Object arg = readWithMessageConverters(webRequest, parameter, parameter.getGenericParameterType());.........}

接下来在readWithMessageConverters中的处理如下：

@Overrideprotected <T> Object readWithMessageConverters(NativeWebRequest webRequest, MethodParameter methodParam,Type paramType) throws IOException, HttpMediaTypeNotSupportedException, HttpMessageNotReadableException {HttpServletRequest servletRequest = webRequest.getNativeRequest(HttpServletRequest.class);ServletServerHttpRequest inputMessage = new ServletServerHttpRequest(servletRequest);Object arg = readWithMessageConverters(inputMessage, methodParam, paramType);if (arg == null) {if (methodParam.getParameterAnnotation(RequestBody.class).required()) {throw new HttpMessageNotReadableException("Required request body is missing: " +methodParam.getMethod().toGenericString());}}return arg;}

在父类AbstractMessageConverterMethodArgumentResolver的方法readWithMessageConverters中的处理如下：简单来说就是根据提供的实现的HttpMessageConverter来对提交的RequestBody中的数据进行转换处理。

@SuppressWarnings("unchecked")protected <T> Object readWithMessageConverters(HttpInputMessage inputMessage, MethodParameter param,Type targetType) throws IOException, HttpMediaTypeNotSupportedException, HttpMessageNotReadableException {//获取数据类型MediaType contentType;boolean noContentType = false;try {contentType = inputMessage.getHeaders().getContentType();}catch (InvalidMediaTypeException ex) {throw new HttpMediaTypeNotSupportedException(ex.getMessage());}if (contentType == null) {noContentType = true;contentType = MediaType.APPLICATION_OCTET_STREAM;}Class<?> contextClass = (param != null ? param.getContainingClass() : null);Class<T> targetClass = (targetType instanceof Class<?> ? (Class<T>) targetType : null);if (targetClass == null) {ResolvableType resolvableType = (param != null ?ResolvableType.forMethodParameter(param) : ResolvableType.forType(targetType));targetClass = (Class<T>) resolvableType.resolve();}HttpMethod httpMethod = ((HttpRequest) inputMessage).getMethod();Object body = NO_VALUE;try {inputMessage = new EmptyBodyCheckingHttpInputMessage(inputMessage);//选择适合的消息处理器来处理参数for (HttpMessageConverter<?> converter : this.messageConverters) {Class<HttpMessageConverter<?>> converterType = (Class<HttpMessageConverter<?>>) converter.getClass();//判断是否适合处理参数if (converter instanceof GenericHttpMessageConverter) {GenericHttpMessageConverter<?> genericConverter = (GenericHttpMessageConverter<?>) converter;if (genericConverter.canRead(targetType, contextClass, contentType)) {if (logger.isDebugEnabled()) {logger.debug("Read [" + targetType + "] as \"" + contentType + "\" with [" + converter + "]");}if (inputMessage.getBody() != null) {inputMessage = getAdvice().beforeBodyRead(inputMessage, param, targetType, converterType);//处理参数body = genericConverter.read(targetType, contextClass, inputMessage);body = getAdvice().afterBodyRead(body, inputMessage, param, targetType, converterType);}else {body = null;body = getAdvice().handleEmptyBody(body, inputMessage, param, targetType, converterType);}break;}}else if (targetClass != null) {if (converter.canRead(targetClass, contentType)) {if (logger.isDebugEnabled()) {logger.debug("Read [" + targetType + "] as \"" + contentType + "\" with [" + converter + "]");}if (inputMessage.getBody() != null) {inputMessage = getAdvice().beforeBodyRead(inputMessage, param, targetType, converterType);//处理数据，对于简单的String数据使用StringHttpMessageConverter进行处理body = ((HttpMessageConverter<T>) converter).read(targetClass, inputMessage);body = getAdvice().afterBodyRead(body, inputMessage, param, targetType, converterType);}else {body = null;body = getAdvice().handleEmptyBody(body, inputMessage, param, targetType, converterType);}break;}}}}catch (IOException ex) {throw new HttpMessageNotReadableException("Could not read document: " + ex.getMessage(), ex);}if (body == NO_VALUE) {if (httpMethod == null || !SUPPORTED_METHODS.contains(httpMethod) ||(noContentType && inputMessage.getBody() == null)) {return null;}throw new HttpMediaTypeNotSupportedException(contentType, this.allSupportedMediaTypes);}//最终返回处理的结果值return body;}

springMVC默认提供了很多参数和结果值处理器，包括如下：

（1）MappingJackson2HttpMessageConverter

（2）GsonHttpMessageConverter

（3）ByteArrayHttpMessageConverter

（4）ObjectToStringHttpMessageConverter

（5）ProtobufHttpMessageConverter

（6）ResourceHttpMessageConverter

（7）StringHttpMessageConverter

（8）AllEncompassingFormHttpMessageConverter